Acceleration sensors are used in many fields of technology. For example, in the motor vehicle industry they are used to control various devices, such as air bags, ABS braking systems, active suspensions, inertial orientation systems and several engine parts.
Various types of acceleration sensors are known which differ from each other in their construction and principle of operation. In recent years, in addition to conventional sensors made on the macroscopic scale and including mechanical switches sensitive to acceleration, miniaturized planar sensors have also been devised and made available. The miniaturized sensors are made using techniques typical for semiconductor devices, such as the planar technology. In many cases, they are formed on a substrate of semiconductive material together with the circuits and electronic components necessary for the amplification and processing of the signal produced thereby. These latter sensors have many advantages over conventional ones, as well as the obvious ones of smaller size, lower cost, greater reliability, improved signal-to-noise ratio, integrability with processing and memory devices, better reproducibility, etc.
The methods of manufacture of planar sensors are based on processing a wafer of semiconductor material, typically silicon, on both of its faces. This is typically done by means of anisotropic etching steps (bulk micromachining), or on only one face by deposition and selective removal of thin films and isotropic etching steps (surface micromachining). This latter type of working or processing is particularly suitable for integration of the structure of the sensor with the processing circuits.
With regard to the operation of planar sensors, this is based on the effect of the acceleration on a structure which has a so-called seismic mass, and which is anchored to the substrate by a mechanical suspension element. The seismic mass, when subject to an inertial force due to an acceleration, moves relative to the substrate, possibly deforming and causing stress in the suspension element. The movement and/or deformation and/or stress are converted into an electrical signal which is then amplified and processed as necessary.
In a known sensor, the seismic mass is provided by one electrode of a capacitor, and wherein the second electrode is in the substrate. A movement of the seismic mass causes a variation in the capacitance of the capacitor which, in turn, is detected and processed by a suitable circuit.